UPDATE (she was cheating): AIO for thinking my gf might be cheating
tldr: My (25m) in-person gf (24f) of 4 years accidentally texted me saying she was going to fly to see me (to give me kisses). I got confused because she wouldn't need to fly to give me kisses and when she explained her side, she told me she thought she was replying to her platonic childhood female friend who I have never heard of (she often tells me about all her friends and their gossip). The more I thought about her story the more it didn't make sense so she tried to gaslight me and I talked myself into believing her despite my gut feeling telling me otherwise.
Thanks everyone who helped me in my original post, in hindsight I seem delusional but I think I was really in shock and trying to justify her story in my head. I left halfway through work today to go to her house after she started ignoring my texts/calls (turns out she did block me temporarily). I packed up all her stuff in my car before I visited her in case she confessed. I tried to have a calm conversation with her about it and she got mad saying I was accusing her of cheating. When I asked to see any proof that "Penny" existed she freaked out when I suggested specific ways to prove it, started crying and admitted to actively cheating on me with 2 people. The one she thought she sent the text to lives in another state and she met him through her friend friend while he was visiting our city last year. During this time they went on a few dates and would make out bit it never went further than that, and some of her friends knew about it. The other was online only but she was sexting him on facetime and sending nudes. I got all my stuff from her house so I didn't need to go back later and after I left she unblocked me and sent me "You're throwing this away for what" and spamming lots of pics of us, my screenshots here start right after the many pics of us. I ended up blocking her on everything right after I sent that final text and plan on going no contact.
She is already telling our mutual friends that I cheated so I'm dreading the drama that might happen with our friend group. Her friends are texting me saying I'm a douche for wanting to move on so quickly (I don't plan on getting into another relationship for a long time), even the friends who knew she was going on dates with that guy are trying to rationalise it by saying it's not that bad and I'm overreacting. Plus her mum called me to yell at me for cheating but I explained it all to her though I don't think she believes me, but that's fine as I'll likely never see her again. I'm just going to try move on and maybe cut off any friends that cause more drama about it which I'm okay with because all my main close friends believe me. Some of them said they got toxic/controlling vibes from her but didn't want to say anything as I wouldn't have listened or seen what they mean (sounds about right tbh).
I read almost every single comment if not all and it made me realise how subtly toxic our relationship was even though I didn't realise until now because nothing major ever happened. I saw a comment about DARVO and it applied to every time we had conflict, she would cry and then accuse me of something random I wasn't doing. I also saw a comment saying it seems like I'm walking on eggshells and I now see that I was because I always did whatever made her happy even if it was wrong or made me upset just so I could avoid being yelled at. I'm not usually this passive with my friends or at work but I really loved her so I guess I had a soft spot for her? I think I definitely got conditioned over the 4 years to become a yes-man and now I see our whole relationship and our interactions in a different way. I'm absolutely devastated that this happened considering we were about to move in together in 2 months time and I was planning on proposing after a year of living together. I'm just glad I know now rather than later. Thanks again for everyone who talked sense into me, even the harsh ones lol I really did need it.